Watering by the clock is simple, but the soil does not follow a clock. After a rain, a timer-driven sprinkler still runs. In a dry spell, a fixed schedule may not be enough. Soil moisture sensors close that gap by measuring how much water is actually in the ground, so irrigation can respond to real conditions.
On a single lawn, a wired sensor next to the controller may be all you need. On a farm, a golf course, or a city's scattered green spaces, the sensors can be far from power and Wi-Fi. That is where a low-power cellular link such as NB-IoT comes in. This guide explains how soil moisture sensors work, what NB-IoT adds, and what to check before you deploy. It is general information, not agronomic advice for a particular crop or site.
What a soil moisture sensor measures
Most soil moisture sensors estimate volumetric water content: how much of a given volume of soil is water. Wikipedia's article on the topic explains that the direct method means removing a soil sample, drying it and weighing it. Sensors therefore measure water content indirectly, through another property of the soil that changes with moisture, such as electrical resistance, dielectric constant, or interaction with neutrons.
The same source notes that the link between the measured property and real moisture has to be calibrated, and that it can vary with soil type, temperature and electrical conductivity. In practice, a sensor installed in heavy clay can read differently from the same sensor in sandy soil unless it is calibrated for that soil.
A separate class of instruments measures soil water potential, meaning how tightly the soil holds its water, rather than how much water is present. The tensiometer is the classic example. It is a water-filled tube with a porous ceramic cup, buried in the soil, whose vacuum reading shows the soil's matric potential. Tensiometers have long been used in irrigation scheduling to help decide when to water.
Common sensing methods
Wikipedia lists several technologies for estimating volumetric water content:
- Frequency domain reflectometry (FDR): measures impedance by applying a frequency of several hundred MHz to short rods that act as a transmission line.
- Capacitance sensors: use a capacitor formed by a printed circuit board as part of an oscillating circuit. They are similar to FDR but run below 1 MHz and are more prone to noise.
- Time domain reflectometry (TDR): works out the dielectric constant of the soil around the probe by timing a signal along a short transmission line.
- Time domain transmission (TDT): similar to TDR, but the sender and receiver sit at opposite ends of the line.
- Neutron moisture gauges: use the way water slows neutrons to estimate moisture between a source and a detector.
- Soil resistivity: measures how strongly the soil resists current between two electrodes.
Which method is right depends on the accuracy you need, your soil, and your budget. Vendors' accuracy claims should be checked against independent data where it exists.
Why add a network connection?
A sensor only saves water if its reading reaches a decision: either a person checking a dashboard, or an irrigation controller that skips a cycle.
The U.S. EPA's WaterSense program describes soil moisture-based irrigation controllers that detect moisture in the ground and override scheduled irrigation when plants do not need water. EPA says these can be stand-alone controllers or add-on devices for an existing clock-based controller. To earn the WaterSense label, a sensor must reliably prevent or allow irrigation at a preset threshold, and labelled products are independently certified. EPA estimates that installing a WaterSense labelled sensor can save an average home with an automatic irrigation system more than 15,000 gallons of water a year.
In a back garden, the sensor is usually wired to the controller nearby. The case changes when sensors are spread across:
- crop fields far from buildings,
- orchards or vineyards with blocks of different soils,
- golf courses (Wikipedia notes they use soil moisture sensors to prevent over-watering and the leaching of fertilisers), or
- many small municipal sites such as parks, road verges and school grounds.
In these settings, running cable to each probe is impractical. A wireless sensor that reports straight to a network lets you see the whole picture in one place.
What NB-IoT brings
NB-IoT (Narrowband IoT) is a low-power wide-area network technology developed by 3GPP, the body behind mobile standards such as LTE. 3GPP announced the completion of NB-IoT standardization in June 2016, in Release 13. According to Wikipedia, NB-IoT focuses on indoor coverage, long battery life and high connection density, and it uses a single narrow band of 200 kHz.
For soil moisture monitoring this matters in a few ways:
- Small data, infrequent reports. A reading of moisture (and often soil temperature) every so often is a few bytes. Wikipedia gives LPWAN data rates of 0.3 to 50 kbit/s per channel, which is more than enough for this.
- Battery power. Field sensors usually run on batteries or a small solar panel. A radio designed for low power stretches the time between site visits.
- No gateways to build. NB-IoT runs on mobile operator networks. If a local operator offers NB-IoT coverage at the field, each sensor can connect on its own, with no private gateway to install. Wikipedia's LPWAN article notes that this kind of third-party service lets sensor owners deploy devices without investing in gateway technology.
Where NB-IoT may not be the best fit
- No operator coverage. Rural fields are exactly where cellular coverage can be patchy, and NB-IoT is not offered by every operator everywhere. Check coverage at the actual sensor locations, ideally with a test unit.
- Very large private sites. A farm with hundreds of probes in one area may find that a privately run LoRaWAN network, with a few gateways and no per-device subscription, costs less over time. The trade-off is that someone has to install and maintain those gateways.
- Moving equipment. Wikipedia notes that NB-IoT devices moving between cells must disconnect and re-register, while LTE-M supports seamless handover. That rarely matters for a buried soil probe, but it does for sensors on vehicles or moving irrigation rigs.
Placement and installation basics
No network can fix a badly placed sensor. Some general principles:
- Measure where the roots are. Probes are usually placed in the root zone of the crop or turf. Deep-rooted plants may need sensors at more than one depth.
- Represent the area. One probe speaks only for the soil around it. Fields with mixed soils, slopes or irrigation zones generally need a probe for each distinct zone.
- Ensure good soil contact. Air gaps around the probe distort readings. Follow the maker's installation steps.
- Mind the antenna. A probe sits in the ground, but its radio usually needs to be above it. Many field units put the electronics and antenna in a housing on a short post. Crops growing tall around it can change signal strength over a season.
- Calibrate for your soil. Because readings depend on soil type, a calibration for your soil, or at least a check against a known reference, makes the numbers more trustworthy.
Questions to ask before you buy
- What does it measure? Volumetric water content, water potential, or both? Does it also report soil temperature or salinity?
- What sensing method, and what accuracy in soil like mine? Ask for test data, not just a datasheet number.
- Is there NB-IoT coverage at my sites? Ask the operator and test before committing.
- What battery life is claimed, at what reporting interval? Wikipedia's LPWAN article notes that makers claim years to decades of battery life but real-world tests have not confirmed it. Ask for field data.
- What does connectivity cost per device per year? Cellular sensors usually carry a subscription on top of the hardware.
- Can it talk to my irrigation controller? Check whether the platform can trigger or block irrigation directly, or only show readings.
- Can I export my data? Open formats and APIs keep you from being locked into a single platform.
- How are firmware updates and security handled? Ask whether devices can be updated remotely and how long the vendor will support them.
The bottom line
A soil moisture sensor estimates water in the ground from a related property such as dielectric constant or resistance, and its readings must be calibrated for the soil. Adding NB-IoT lets battery-powered probes in remote fields or scattered sites report over a mobile operator's network, without private gateways, as long as there is coverage. Used well, these sensors can help irrigation follow the soil rather than the clock. EPA's WaterSense program estimates substantial water savings from soil moisture-based controllers in home landscapes.
Sources
- Wikipedia, Soil moisture sensor: https://en.wikipedia.org/wiki/Soil_moisture_sensor
- Wikipedia, Tensiometer (soil science): https://en.wikipedia.org/wiki/Tensiometer_(soil_science))
- U.S. EPA WaterSense, Soil Moisture-Based Irrigation Controllers: https://www.epa.gov/watersense/soil-moisture-based-irrigation-controllers
- Wikipedia, Narrowband IoT: https://en.wikipedia.org/wiki/Narrowband_IoT
- Wikipedia, LTE-M: https://en.wikipedia.org/wiki/LTE-M
- Wikipedia, Low-power wide-area network: https://en.wikipedia.org/wiki/Low-power_wide-area_network
- 3GPP, "Standardization of NB-IOT completed" (June 2016): https://www.3gpp.org/news-events/3gpp-news/nb-iot-complete